This invention relates to ceramic compositions having chemical stability and electronic conductivity at elevated temperatures. In particular, it relates to ceramic anode compositions for use in molten carbonate fuel cells and to compositions for use in other reducing environments at elevated temperatures.
Previous molten carbonate fuel cells have included an anode of such as porous nickel, a cathode of such as porous lithiated nickel oxide and molten carbonate as electrolyte retained in a porous tile. A representative electrolyte is a mixture of Li.sub.2 CO.sub.3 and K.sub.2 CO.sub.3. Typical operating temperatures of such molten carbonate fuel cells are between 600.degree.-700.degree. C. and at such elevated temperatures there has been loss of physical stability of the anode structure over extended periods of operation.
Nickel metal as anode material is chemically, but not mechanically stable. A porous nickel anode will creep at the cell operating temperature and the required mechanical loading resulting in reduced porosity. This problem has been addressed by alloying the nickel with chromium or aluminum, for creep resistance but with accompanying increased cost and effort in anode preparation. Other efforts have involved plating ceramic particles with nickel to provide a hard core. Unfortunately, the nickel plating tends not to wet the ceramic and to coalesce into islands of nickel on otherwise uncoated particles.
The present inventors, jointly with Sim, have reported stable, electronically conductive ceramic cathode compositions in U.S. Pat. No. 4,564,567. These compositions involved lithium-transition metal oxygenates with suitable dopants to increase their electrical conductivity to a level for molten carbonate cathode use. At that time, the inventors did not contemplate the use of such ceramic oxygenates as anode materials because the highly reducing fuel gas in contact with the anode was expected to significantly change and structurally degrade such ceramics. In fact, efforts to sinter particulate LiFeO.sub.2 in hydrogen gas at about 1000.degree. C. reduced the ceramic to iron metal which is not chemically stable in the molten alkali metal carbonate fuel cell.
Therefore, in view of the above, it is an object of the present invention to provide an electrically conductive ceramic composition that is stable in fuel gas environments.
It is a further object to provide a ceramic anode composition that is physically and chemically stable in fuel gases and molten alkali metal carbonates at elevated temperatures.
It is a further object to provide an improved method of preparing an anode or other electrical component of high temperature, electrically conductive ceramic.
It is a further object to provide a method of preparing an anode for use in a molten carbonate fuel cell with improved electrical conductivity not dependent on the addition of a doping additive.